"User defined breakpoint" was new to me to me, so I googled it and it seems to come up when dlls or files get mixed up. Install any new Microsoft or other video tools? Any new codecs?
Reinstalled Vegas a few times.
This turn out to be a Raylight Ultra issue.
Was using older version.
Updated at the bequest of the team in Austin, and all is rectified.
The guys at DVFilm were all over this and had a solution withing an hour.
Very impressed!